The present invention relates to a cathode for a cold cathode gas discharge device. The present invention has particular application to a cold cathode apparatus and methods for the construction and operation of gas lasers.
Cold cathode gas lasers utilize an electrical gas discharge in which ionized species are accelerated toward the cathode by an electrical field. When the ions strike the cathode, electrons are ejected from the cathode. This produces the electrical operation of the cathode/anode structure and the resulting light emitting gas discharge within a gas discharge confinement bore extending between the cathode and the anode.
There is a tendency for some of the ions which are accelerated toward the cathode to become buried in the cathode.
There is also a sputtering effect in which the impacting ions knock out some of the cathode material. This sputtered material diffuses until it contacts some surface of the laser. Gas can be trapped beneath the sputtered material when the sputtered material settles back onto a surface of the laser.
Gas lasers operate at low gas pressures, about a few torr. After some period of time of operation, the gas lost because of the ions buried in the cathode and entrapped by the sputtering action can cause a drop in the gas pressure of the laser and a drop in the power output of the laser.
The cathode constructions used with prior art lasers have presented problems in obtaining a predictable operating life for the laser. Some prior art gas lasers might operate satisfactorily for only about 5,000 hours while others would operate satisfactorily for 30,000 or more hours.
Since sputtering, and the resultant entrapment of gas in the gas laser, cannot be eliminated, the prior art apparatus and methods have attempted to solve the problem by trying to control the rate of sputtering. Various materials have been used for the cathode. Aluminum is commonly used because it is not too expensive, it is not too dirty, and it is easy to oxidize. Oxidized aluminum produces a surface layer which has a relatively low sputtering rate.
The loss of gas, and gas pressure, as described generally above, is referred to as "gas cleanup" in the gas laser art.
There is another effect which occurs during the operation of the gas laser and which results in the release of gas previously entrapped in the cathode. In this gas release mode the impacting ions are effective to release gas atoms or molecules that have been previously entrapped. The impacting ions do this by sputtering cathode material above or beside the entrapped gas atoms or molecules.
The conventional prior art thinking had been that it was necessary to have a relatively large surface area for the active surface of the cathode in order to minimize hot spots. That is, a large surface area had been considered to be useful and necessary in order to produce a low current density to minimize sputtering and gas clean-up. Prior art cold cathode structures often incorporated long cylindrical tubes with the end of the bore positioned within the interior of the cylindrical cathode tube.
It was discovered, in the present invention that, during operation of this prior art structure, some parts of the cathode operated at a higher current density than other parts of the cathode. The higher current density areas of this prior art cathode construction sputtered more material than the lower current density areas. Gas was trapped by the sputtered material over a broader area than the area of higher current density itself because the material sputtered from the higher current density area diffused into areas of lower current density. During continued operation of the laser less of the trapped gas in the lower current density area was subsequently released because the area was a lower current density area and therefore released less gas in the release mode. The result was permanent trapping of gas because of the non-uniform current density over the active surface of the prior art cathodes.